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The present invention relates generally to methods and apparatus for separating liquid products from a slurry comprising solid particles and liquids. More specifically, the present invention relates to methods and apparatus for separating liquid products from a slurry used in a Fischer-Tropsch slurry bubble column reactor.
A Fischer-Tropsch reaction generally entails contacting a stream of synthesis gas (hydrogen and carbon monoxide) with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons. More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes, olefins, and oxygenated hydrocarbons or oxygenates. Research continues on the development of more efficient Fischer-Tropsch catalyst systems and reaction systems that increase the selectivity for high-value hydrocarbons in the Fischer-Tropsch product stream.
Originally, the Fischer-Tropsch synthesis was operated in fixed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated reactors, sometimes called xe2x80x9cslurry reactorsxe2x80x9d or xe2x80x9cslurry bubble columns,xe2x80x9d operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are converted to gaseous and liquid products. As gaseous products are formed, they enter the gas bubbles and are collected at the top of the reactor.
Because of the formation of liquid products (commonly called waxes in this context), it is necessary to maintain the slurry at a constant level by continuously or intermittently removing liquid products from the reactor. One problem with the removal of liquids, however, is that catalyst particles are dispersed in the liquid and must be separated from the slurry and, in some cases, returned to the reactor in order to maintain a constant inventory of catalyst in the reactor. Several means have been proposed for separating the catalyst from the liquid products, e.g., centrifuges, sintered metal filters, cross-flow filters, magnetic separators, gravitational settling, etc.
Filtration is one of the catalyst-liquid separation methods used with Fischer-Tropsch reactors. Filtration techniques are characterized by solid-liquid separation systems that remove liquid products from a slurry by drawing the fluid across a filter medium. The filter medium may be simply a filter substrate or may be composed of a filter cake disposed on a filter substrate, such that the filter cake forms a primary filter. A filter cake is formed as solid particles are deposited on the filter substrate creating a permeable barrier between the slurry and the substrate. The thickness and permeability of the filter cake is critical to the efficient operation of the filtration system.
In a commercial slurry bubble column reactor, the severe hydrodynamic conditions inside the reactor, coupled with the desired long lifetime of the catalytic material, typically results in catalyst attrition. As the catalyst breaks down over time, sub-particles of various sizes may be created, including very small particles known as xe2x80x9cfines,xe2x80x9d some of which may even be sub-micron in size. The presence of fines in the reactor tends to greatly reduce the effectiveness of the catalyst-liquid separation system.
In a catalyst-liquid separation system utilizing filtration, cycle time between backwashing operations, as well as filter life, may be greatly reduced because the fines tend to reduce the permeability and flux of the filter system. Likewise, centrifuges and gravitational settlers have been found unsuccessful in reducing the percentage of fines because the fine particles are so small that they will not settle out of the liquid solution in a practical amount of time, if at all. Magnetic separation has been similarly ineffective in removing catalyst fines from the slurry. Thus the performance of a catalyst-liquid separation system has hereto been undesirably dependent upon the age of the catalyst. For example, when the catalyst is new the catalyst-liquid separation system operates at a very high rate that decreases as the catalyst breaks down.
Thus, there remains a need in the art for methods and apparatus to maintain the effectiveness of a catalyst-liquid separation system independent of the age or degree of attrition of the catalyst. Therefore, the embodiments of the present invention are directed to methods and apparatus for removing catalyst fines from a slurry that seek to overcome the limitations of the prior art.
Accordingly, there are provided herein methods and apparatus for separating liquid products and catalyst fines from a slurry used in a Fischer-Tropsch reactor. The preferred embodiments of the present invention are characterized by a settling system that continuously or intermittently removes catalyst sub-particles fines from the slurry by way of a sub-particle rich stream, coupled with a distinct separation system that separates the slurry into a catalyst-rich stream and a catalyst-lean stream that supplies most of the commercial products of the reactor system. The embodiments of the present invention act to reduce the overall concentration of catalyst fines in the slurry, thereby increasing the effectiveness and/or the life of a catalyst-liquid separation system.
One preferred embodiment includes a slurry bubble column reactor system having a first circulation loop with a catalyst-liquid separation system that separates the slurry into a catalyst-rich stream and a catalyst-lean stream. The catalyst-lean stream provides a stream from which most of the products of the reactor can be extracted. In this embodiment, the reactor system also includes a second circulation loop that, in some preferred embodiments, comprises a settling chamber, which segregates at least a portion of catalyst sub-particles from catalyst particles. A sub-particle lean stream and a sub-particle rich stream can then be extracted from the settling chamber. The sub-particle rich stream, which may contain a portion of the liquid products, can then be removed from the system, and may be further processed if desired to recover some of the liquid products. It will be understood that the terms xe2x80x9crichxe2x80x9d and xe2x80x9cleanxe2x80x9d are relative terms, so that, for example, xe2x80x9csub-particle rich streamxe2x80x9d refers to a stream containing a higher proportion of sub-particles, as compared to other particles, than other streams in the system. The overall concentration of catalyst fines in the slurry is maintained at a reduced level, thereby increasing the effectiveness and life of the catalyst-liquid separation system.
In a second preferred embodiment, a slurry bubble column reactor system has only one circulation loop, which includes both a settling chamber and a catalyst-liquid separation system. The slurry moves through the settling chamber, from which is extracted a sub-particle rich stream that is removed from the system. The sub-particle lean stream can then be processed in the catalyst-wax separation system to produce a catalyst-rich stream and a catalyst-lean stream. Production quantities of liquid products can then be collected from the catalyst-lean stream. In addition, some liquid products can be recovered from the sub-particle rich stream after further processing.
The present invention may also be embodied as a method for removing solids from a slurry by circulating the slurry through a settling chamber to produce a sub-particle lean stream and circulating the slurry through a catalyst-liquid separation system to produce a catalyst-lean stream. The preferred embodiment may also include removing the sub-particle lean stream and the catalyst-lean stream from the system.
Thus, the present invention comprises a combination of features and advantages that enable it to substantially reduce the concentration of catalyst fines in the slurry, thereby increasing filter effectiveness and life. These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings.